This invention relates to an improved process for the preparation of nitrile polymers wherein 1,4-cyclohexadiene or a selected derivative thereof is used as chain transfer agent. More particularly, this invention involves the polymerizaion of nitrile polymers having improved color by using as chain transfer agent a cyclohexa-1,4-diene derivative having the formula: ##STR1## wherein R.sup.1 and R.sup.2 each independently represent hydrogen or an alkyl group of 1 to 4 carbon atoms and each R independently represents an alkyl group of 1 to 4 carbon atoms or one or two pairs of R groups together may represent one or two fused benzo groups and n is an integer of 0 to 4.
The tendency of certain free radical polymerizable materials to polymerize to very high molecular weight polymers under normal polymerization conditions is already well known. These very high molecular weight polymers generally have the disadvantage of being difficult to process and often require the use of special processing equipment or temperatures so high as to have a detrimental effect on the strength and color of the processed polymer.
Different techniques have been proposed for regulating the molecular weights of such polymers so as to make them more easily processable. For example, the polymerization temperature may be raised, but this increases the danger of runaway polymerization. Another technique involves conducting the polymerization in an organic diluent but while effective as a means of lowering molecular weight, it has disadvantages of increased cost and inconvenience of separating the polymer from the organic diluent. Still another technique involves the use of a chain transfer agent or molecular weight regulator. A variety of chain transfer agents have been used with varying effectiveness. For example, mercaptans; terpenes such as terpinolene and dipentene; cyclo-olefins such as cyclohexene, 1-methylcyclohexene-1 and 1,2 carboxy-5-methylcyclohexene-4; Eugenol, etc. have all been used as chain transfer agents.
In receent years it has been discovered that certain polymeric nitrile polymers particularly those having a high proportionate amount of olefinic nitrile component are especially suitable for packaging and other applications because of their excellent water and oxygen barrier properties. In the polymerization of such polymers the necessity for molecular weight control is evident but the development of undesirable color in the polymers during processing is a serious problem.